A number of components in wind turbines, such as the rotor, the nacelle or the tower, are usually subjected to asymmetric loads. Asymmetric loads are produced due to variable conditions of the wind and add to the action of the vertical and/or horizontal wind loads as well as loads caused by the rotor when in operation. Asymmetric loads result in fatigue damage on the turbine structure. This is highly relevant in current wind turbine structures due to their great overall size with increasingly larger blade spans. This in practice results in a shorter lifetime and/or a more frequent maintenance of at least the above mentioned wind turbine components.
It is known in the art to use asymmetric load control means. Such control means perform, based on the asymmetric loads, a cyclic actuation on the wind turbine blade pitch system to control the aerodynamic rotational torque of the rotor. This can be carried out, for example, through a PID controller to reduce or mitigate the effect of the asymmetric loads acting on the wind turbine components. Such asymmetric load control means include measuring means for measuring displacements or deflections of certain elements of the wind turbine. Known measuring means in this field are, for example, sensors placed at the root of the blades or sensor means adapted for measuring deflections in the wind turbine low speed shaft. This is disclosed, for example, in documents WO2005010358 and WO2008087180.
The main disadvantage of the use of the above mentioned control means is that the displacements to be measured in certain elements of the wind turbine are usually very small. In addition, it has been found that the measured displacements may be caused by other loads apart from asymmetric loads.
As a further example, document U.S. Pat. No. 8,240,990 provides time based correction of the initial conditions during shutdown for reducing asymmetric loads on wind turbine components. The method disclosed comprises calculating a time delay for pitching each blade toward feather upon initiation of a shutdown condition. According to each blade angle, a blade is moved toward feather with an initial or final pitch rate until all the blades have reached approximately an identical blade angle. The blades are then moved simultaneously together to feather at the final pitch rate.
It has been however shown that the pitch control may not be capable of reducing harmonic loads caused by asymmetric loads on the structure. For this reason, the prior art has also tried to use methods for reducing asymmetric loads without adjusting pitch are known. For example, document U.S. Pat. No. 7,118,339 discloses a method for reducing loads in a wind turbine having a rated wind speed. The method consists in operating the wind turbine above a selected wind speed and introducing a yaw error.
The above prior art methods penalise energy production so there is still the need for a more efficient strategy to at least mitigate the increased loads on a wind turbine structure at reduced costs while still maintaining the power produced at a desired level.